The addition of red phosphorus as a flame retardant has been recommended for a variety of plastic materials. For example, Largman and Stone, in U.S. Pat. No. 3,847,861, disclose that the incorporation of red phosphorus into filled or unfilled polyesters, as for example poly(ethylene terephthalate), provides a polymer having good physical properties and flame retardance. Similarly, Largman and co-workers in U.S. Pat. Nos. 3,086,488 and 3,882,070 disclose that polyamides, such as Nylon 6 and Nylon 66, are flame retarded by red phosphorus. Red phosphorus has also been reported as a flame retardant in other polymeric materials, as for example, polyolefins (U.S. Pat. No. 3,931,101); polyphenylene oxides (U.S. Pat. Nos. 3,663,054 and 3,974,235); polyacetates (U.S. Pat. No. 3,884,867); ABS Resins (Canadian Pat. No. 846,892); polycarbonates (Japan Patent application Nos. 73/85,634 and 75/113,475); poly(chlorostyrene; polyacrylates (U.S. Pat. No. 3,125,553); vinyl polymers (U.S. Pat. No. 3,551,379); natural synthetic elastomers (U.S. Pat. No. 3,597,385 and S. Africa Pat. No. 700,200); epoxy resins (U.S. Pat. No. 3,375,135); polyurethane (U.S. Pat. No. 3,763,057); and polyimides (German Pat. No. 2,610,306).
While red phosphorus is very effective as a flame retardant for such polymeric materials, its use results in adverse effects, which in some cases seriously limits its usefulness. One extremely serious disadvantage is its tendency to form phosphine at elevated temperatures which usually accompany the processing of polymeric materials; and by reaction with atmospheric humidity. Phosphine is spontaneously flammable, is highly toxic by inhalation and is a strong irritant. Human tolerance levels as designated by the U.S. Occupational Safety and Health Administration (OSHA) are 0.3 ppm in air or less, and as disclosed in U.S. Pat. No. 3,883,475 amounts of 400 ppm or more are dangerous to life. This problem is especially critical if high temperatures are employed in polymer fabricating processes, and if a polymer made flame proof by red phosphorus is maintained or shipped in closed containers since in such instances these concentration levels may be considerably exceeded.
Various methods have been reported in the literature for minimizing phosphine evolution. One such method which is described in U.S. Pat. Nos. 4,210,630 and 2,035,953, involves the precipitation of metal hydroxides upon the outer surface of red phosphorus particles. These procedures suffer from the disadvantage that useful hydroxide salts are often expensive. Adverse effects also include poor filterability of the phosphorus containing suspension, and in an unsatisfactory stability of the red phosphorus treated therewith.
Another attempt to avoid phosphine formation consists in the encapsulation of the phosphorus particles, as recommended in German Pat. Nos. 2,734,103 and 2,754,491. This method however is expensive and does not work well by itself requiring the use of a combination of phosphine suppressing agents. U.S. Pat. Nos. 3,883,475 and 4,187,207 disclose the use of so-called phosphine traps, in which such compounds as cupric oxide are employed to fix or trap any phosphine produced. Cupric oxide considerably reduces the amount of phosphine released into the atmosphere but it accelerates the oxidation rate of red phosphorus. This results in the formation of phosphorus derived acids which have a deleterious effect upon the physical properties of the polymer substrate. As is mentioned in European Pat. No. 6549, small quantities of acids of one-to five-valent phosphorus are also formed when red phosphorus-containing polymers are processed at high temperature in contact with oxygen. During die casting or injection molding, these compounds accummulate at the surface of the plastic articles produced and reduce their resistance to current leakage making them less useful for electrical or electronic applications.
It has now been discovered that metal iodides serve as effective phosphine suppressants while at the same time reducing the oxidation of phosphorus to its acid derivatives. It is the purpose of this invention to provide a thermoplastic composition which offers the excellent flame retardance produced by the addition of red phosphorus while avoiding the deleterious side effects of phosphorus addition.